1. Field of the Invention
This invention relates to the use of special, substantially clear, aqueous and at least largely solventless polyurethane dispersions as universal household adhesives and to a process for their production.
Universal household adhesives, also known as multipurpose adhesives, are used for bonding a number of substrates encountered in the home (paper, cardboard, photographs, fabrics, leather, felt, bast, cork, films, metals, such as aluminium and iron, china, ceramics, glass, wood, various plastics, including for example polystyrene foams). The adhesives in question are expected to produce an adequate adhesive effect on these various substrates which differ chemically and physically in their surface structure and which are normally subjected to a special surface treatment before bonding.
Compared with the large variety of classes and types of adhesive used in industry and workshops, there are only a few substances which are capable of meeting the stringent demands imposed on the universality of a multipurpose household adhesive. Among those substances, only polyvinyl acetate and its copolymers are widely used--normally in solution or, for glueing wood, in the form of a dispersion.
The demand for universality represents a particularly difficult selection criterion for an adhesive. Ultimately, it means that the adhesive molecules must show equally high affinity for polar and apolar interfaces. Accordingly, the statement that a certain substance is suitable as an adhesive does not indicate to the expert whether it can also be used as a universal household adhesive.
In addition to the universality requirement, there has also recently been a demand for solventless, physiologically safe, clear, aqueous formulations of neutral odor in the field of universal household adhesives. However, these formulations are intended at the same time to lead to adhesives of which the dried films in turn have a certain resistance to water. In addition, these water-based adhesives are also intended to be able to bond substrates that are difficult to bond, such as plastics. They are also intended to have a long storage life.
This requirement profile could not be fully satisfied either on the basis of the binders hitherto preferred for universal adhesives, namely polyvinyl acetate and vinyl acetate copolymers, or by such alternatives as nitrocellulose. Although polyvinyl acetate can be produced without solvents in the form of aqueous dispersions, the dispersions obtained are not transparent, but milky white. They show good performance properties when used, for example, as wood glue. The acrylates and styrene acrylates widely used as dispersion adhesives are also not known on the market in the form of transparent household adhesives with the properties mentioned.
It is known that specially selected embodiments of the aqueous polyurethane dispersions known for decades are suitable as multipurpose universal adhesives. Polyurethane dispersions consist of adducts of polyfunctional isocyanates (isocyanate component) with polyfunctional OH compounds (polyol component) which contain co-condensed units capable of salt formation in aqueous solution. It has surprisingly been found that polyurethane dispersions in which the polyol component is based on polypropylene glycol and the isocyanate component is based on tetramethyl xylene diisocyanate, are also suitable as universal household adhesives and provide good adhesion values. It has also surprisingly been found that dispersions of the type in question can be produced without using inert solvents.
2. Discussion of Related Art
DE-OS 36 30 045 broadly describes an adhesive based on an aqueous dispersion of polyurethanes containing incorporated carboxylate and/or sulfate groups of which the basic diisocyanate component consists of a mixture of at least two (cyclo)aliphatic diisocyanates. The DE-OS mentions numerous polyols, numerous isocyanate compounds and numerous salt-forming components as modifying agents. It also mentions the chain-extending agents typically used in the field in question. The adhesives are said to be suitable for bonding any substrates.
The broad disclosure of DE 36 30 045 in regard to the polyol component encompasses both polyurethane dispersions, in which polyesters are used as the OH-functional component, and also those in which polyethers are used. However, polyurethane dispersions based on polyurethanes synthesized from OH-functional polyesters are not suitable as universal household adhesives because they undergo hydrolysis in storage and hence do not have the required stability in storage.
Although DE 15 95 602 also mentions polymerization products of propylene oxide, for example, as a possible polyol, there is no reference to the fact that polyols such as these are a suitable basis for polyurethane dispersions for universal household adhesives having the requirement profile mentioned above.
Although universal household adhesives based on polyurethane dispersions which have the stated requirement profile are already known from DE 38 27 378, the dispersions in question are not dispersions in which the polyurethanes are formed by reaction of polypropylene glycol with isocyanates.
Although polymerization products of propylene oxide are mentioned among many other polyethers in DE 38 27 378 and in the corresponding EP 354 471, they are only mentioned as an addition to the described polyol mixture consisting of polytetrahydrofuran. At the same time, it is pointed out in both patents that polyurethane dispersions based on polyethylene oxide and/or polypropylene oxide as the OH-functional component are also unsuitable as multipurpose adhesives because they show poor adhesion to plastic surfaces and, accordingly, do not satisfy the universality requirement. This is consistent with the statement in DE-OS 17 69 387 that polyether diols which have been prepared from oxides, such as ethylene oxide or propylene oxide, are unsuitable as polyol component for the production of polyurethane dispersions intended for the bonding of plasticized PVC plastics.
Among many other suitable isocyanates, EP 354 471 also mentions tetramethyl xylene diisocyanate, although there is no reference to the fact that the combination of this isocyanate with a polyol component based on polypropylene glycol leads to polyurethane dispersions which, when used as multipurpose adhesives, do show good adhesion, for example to plastic surfaces. In addition, there is no reference to the fact that, where this combination is used, the dispersion can be prepared without inert solvents. Instead, the acetone process is preferred in both the cited patents.
Accordingly, the problem addressed by the present invention was to show that special, aqueous, transparent polyurethane dispersions based on a polyol component which, in turn, is based on polypropylene glycol satisfy the complex and partly conflicting requirements mentioned above in regard to universal household adhesives. These requirements also include very good stability to hydrolysis and high adhesive strength. The invention also sought to provide a process by which it would be possible, in contrast to the prior art, to produce polyurethane dispersions suitable as domestic multipurpose adhesives without having to use inert solvents, so that the dispersions could even be produced without any residual solvent content whatever.